Ink jet recording is an image forming technique comprising ejecting ink droplets from fine nozzles of a recording head to form letters or figures on a recording medium such as paper. Ink jet recording technologies that have been implemented include drop-on-demand techniques, such as a method in which electrical signals are converted to mechanical signals by a piezoelectric actuator to discontinuously eject ink droplets stored in the nozzle head and a method in which part of ink is rapidly heated to generate a bubble near the nozzle of a recording head to discontinuously eject ink droplets by the volume expansion of the bubble.
Aqueous pigment inks having a pigment dispersed in water have recently been developed for use in ink jet recording systems. Compared with inks containing water-soluble dyes, aqueous pigment inks are characterized by excellent water resistance and light resistance. It is a generally followed practice that such aqueous pigment ink is prepared by dispersing a pigment in an aqueous medium with the aid of a dispersant, such as a surface active agent or a polymeric dispersant.
For example, JP-A-3-157464 proposes using a polymeric dispersant for helping pigment particles be dispersed and using a mixture of water, a non-volatile organic solvent, and a lower alcohol as an aqueous medium in a pigment ink formulation containing an acetylene glycol-based penetrant thereby to secure dispersion stability of the pigment. However, use of a dispersant for pigment particles leads to an increased number of elements involved in ink preparation, which makes it difficult to control ink physical properties such as viscosity within designed ranges. Besides this, the proposed pigment ink still has the problem that a sufficient image density is hard to obtain.
Further, aqueous pigment ink containing a dispersant has the dispersant merely adsorbed on the surface of pigment particles. Since a strong shear force is applied to ink ejected through a fine nozzle of a recording head, the dispersant can come off the surface of pigment particles to lose its dispersing capability, which can result in instable ink ejection. Destabilization can also occur when the aqueous pigment ink is stored for a long time.
Introducing a sulfonic acid group on the surface of pigment particles has been proposed as a successful technique for dispersing pigment particles in water. For example, JP-A-10-110129 discloses a pigment ink containing a surface-sulfonated organic pigment which is obtained by treating an organic pigment dispersed in a solvent having no active protons with a sulfonating agent (referred to as related art (1)). The inventors of the related art 1 allege that the pigment ink has excellent dispersion stability and exhibits satisfactory ejection stability from the nozzle of a recording head (the property of being stably ejected from the recording head in a fixed direction).
JP-A-11-49974 teaches that a positively chargeable organic pigment mass is prepared by treating a sulfonated organic pigment mass with a monovalent metallic ion and that a pigment prepared from the positively chargeable organic pigment mass is formulated together with a dispersant and water into an aqueous ink composition having high storage stability (dispersion stability) (referred to as related art (2)).
Although the ink compositions of the related arts (1) and (2) which contain surface-treated pigment particles as a colorant are excellent in dispersion stability and ejection stability as compared with conventional ink jet recording inks, the images printed with them on recording media such as plain paper or inkjet recording media (recording media having an ink receiving layer designed for ink jet recording ink) have insufficient abrasion resistance. This seems to be due to insufficient fixability of the surface-treated pigment particles on the recording media.
Application of polymer-encapsulated pigments to ink jet recording technologies is known for improved fixability of pigments on a recording medium. JP-B-7-94634 and JP-A-8-59715 disclose encapsulated pigment particles. JP-A-5-339516, JP-A-8-302227, JP-A-8-302228, and JP-A-8-81647 propose pigment particles having a polymer grafted on their surface. JP-A-5-320276 suggests a method of microencapsulating hydrophobic powder with an amphiphilic graft polymer, which has turned out to involve the problem that use of such a previously polymerized material for encapsulation results in too large a particle diameter. Additionally, JP-A-8-218015, JP-A-8-295837, JP-A-9-3376, JP-A-8-183920, JP-A-10-46075, JP-A-10-292143, JP-A-11-80633, JP-A-11-349870, and JP-A-12-7961 disclose inks containing pigment particles coated with a resin capable of forming film at room temperature by phase reversal of emulsion. JP-A-9-31360, JP-A-9-217019, JP-A-9-316353, JP-A-9-104834, JP-A-9-151342, JP-A-10-140065, JP-A-11-152424, JP-A-11-166145, JP-A-11-166145, JP-A-11-199783, and JP-A-11-209672 propose inks containing pigment particles coated with an anionic group-containing organic polymer by acid precipitation. JP-A-9-286939, JP-A-12-44852, JP-A-12-53897, JP-A-12-53898, JP-A-12-53899, and JP-A-12-53900 propose inks comprising a polymer emulsion having polymer particles impregnated with a colorant by phase inversion of emulsion (referred to related art 3). Even in ink compositions containing such a colorant as obtained by phase inversion of emulsion or acid precipitation, the polymer adsorbed on the pigment particles can separate and dissolve in ink, resulting in insufficient performance in terms of dispersion stability, ejection stability, image quality, and the like, which depends on the kind of organic solvents used as, for example, a penetrant, though. Because the ink according to the related art 3 not a little suffers from separation of polymer from pigment particles, the pigment content in ink is of necessity limited for assuring dispersion stability. As a result, the ink only provides recorded matter with low image densities and, when printed on plain paper, easily feathers and attains only low color developability.